Various metal materials such as stainless steel, common steel, aluminum, aluminum alloys, copper, and galvanized steel are widely used in various fields, such as building materials and electronic devices, for their excellent corrosion resistance and external appearance. When these metal materials are used as structural members, various components, etc., it is often the case that metal material plates need to be joined together or to other components or members. In this case, metal material plates are conventionally often joined together by welding.
However, when metal materials are joined by welding, there is a problem that weld marks are left on the surfaces of welded metal material plates; therefore, excellent external appearance unique to metal material plates is impaired. Further, sheet-metal processing is required to remove weld marks or weld distortions. However, such sheet-metal processing is avoided by workers as well as peripheral people because a lot of time and effort is required and working conditions are worsened by noise generation etc.
For this reason, as a method for joining metal materials as an alternative to welding, a bonding method using an adhesive has received attention in recent years. The bonding method using an adhesive is advantageous in that external appearance of metal materials is hardly impaired; therefore, the above-described sheet-metal processing is not required. However, the surface of a metal material is generally often coated with a stable oxide film. Particularly, stainless steel is coated with an oxide film excellent in corrosion resistance but very low in affinity for an adhesive, and therefore has a problem of poor adhesion. Further, the bonding method has a problem that an adhesive interface is poor in water resistance; therefore, when an adhesive joint between metal materials is exposed to a high-temperature and high-humidity atmosphere, bond strength is significantly reduced in a short period of time.
The affinity of such a metal material for an adhesive, especially for an epoxy-based adhesive can be improved by previously subjecting the surface of the metal material to activation treatment using an acid. For example, a method is known in which the surface of a stainless steel plate is treated with a mixed aqueous solution of sulfuric acid and oxalic acid. Another method is also known in which an aluminum plate or an aluminum alloy plate is immersed in a phosphoric acid aqueous solution or a dichromic acid aqueous solution or electrically anodized while being immersed in such an aqueous solution. These treatment methods are known to develop excellent adhesiveness, and are therefore practically used in, for example, the process of assembling an airplane.
However, the above-described method for activating the surface of stainless steel by acid treatment has a problem that smut is generated on the surface of stainless steel. This smut can be removed by treating the surface of stainless steel with a mixed aqueous solution of dichromic acid and sulfuric acid. However, such desmutting treatment generates chromium-containing wastewater, and is therefore strictly limited from the viewpoint of environmental destruction.
As a bonding method not requiring such desmutting treatment, a method has recently been tried in which a primer is previously applied onto the surface of a stainless steel plate to form an organic thin film (primer layer) to improve adhesiveness. For example, Patent Literature 1 discloses a method in which a stainless steel plate is treated with an aqueous primer containing an acidic phosphoric acid ester and/or a salt thereof and water to improve the adhesiveness of the stainless steel plate.
Further, the use of a silane coupling agent is known as a technique for imparting adhesiveness to a stainless steel plate, a common steel plate, an aluminum plate, an aluminum alloy plate, or the like. For example, Patent Literature 2 discloses a method in which the surface of a common steel plate or stainless steel is treated with a silane-based coupling agent to improve its adhesiveness to a fluorine-based coating film.
Non-Patent Literature 1 states that the adhesiveness of a stainless steel plate, a common steel plate, an aluminum plate, an aluminum alloy plate, or the like is improved by treatment using a silane coupling agent having a functional group reactive with an epoxy-based adhesive. Further, Patent Literature 3 discloses a primer composition containing a multifunctional epoxy resin and a bisphenol A-type epoxy resin as main components and imidazole as a curing agent.
With the spread of the bonding method in recent years, there are more and more cases where adhesive joints are temperature and high-humidity environment. For example, in the case of immersion in boiling water, that is, exposure to the severest conditions, it is difficult for surface treatment using a silane coupling agent or the like to suppress a reduction in the adhesiveness of adhesive joints. Therefore, Non-Patent Literature 2 proposes silicoater treatment as treatment for imparting stable adhesiveness to adhesive joints of various metal materials even in boiling water.